Antibody titers to simian hemorrhagic fever (SHF) virus were determined by enzyme-linked immunosorbent assay in 285 patas monkeys (216 feral; 69 laboratory-reared). Antibody to SHF virus was detected in 48.6% of the feral animals, but antibody was not detected in any of the laboratory-reared animals. Transmission studies showed that 9 of the 216 feral animals were long-term asymptomatic chronic carriers of SHF virus. Antibody titers in the chronic carriers varied from no detectable antibody to a titer of 6000. At birth, babies of patas monkeys had the same antibody titers as their mothers. Maternally acquired antibody decreased gradually and was no longer detectable about 3 months after birth. Babies born to chronically infected mothers also lost their maternal antibody within this time frame and no evidence of in utero infection was found. These results suggest that SHF virus infection of patas monkeys is by horizontal means. An in vitro test has been developed to detect monkeys chronically infected with SHF virus.